1. Field of the Invention
This invention relates in general to communications antennas, and, more particularly, to a mobile communications antenna for use over a selected band of frequencies in the VHF and UHF frequency bands.
2. Description of the Related Art
It has long been known that an antenna can be mounted on a pane of glass and that the dielectric properties of the glass can be advantageously used to capacitively couple the antenna to radio apparatus when they are on opposite sides of the glass.
The first teachings of mounting such an antenna on a non-conductive surface of a vehicle can be found in such patents as U.S. Pat. No. 1,715,952 to J. A. Rostron. Rostron taught a window mounted antenna that was capacitively coupled, through the window, to a transmitting or receiving apparatus.
With the popularity of radios in automobiles, several early inventors patented antennas which were mounted on vehicular windows or windshields. While most of these early references were directed towards antennas suitable for receiving signals, it was the recent popularity of first the Citizen's Band radios and, more recently, the cellular telephone, as a car accessory, that caused the prior art to expand in the area of mobile communications antennas suitable for mounting on non-conductive areas of a vehicle that could both transmit as well as receive signals.
The cellular telephone system generally employs for each subscriber to the system, a transceiver operating in the VHF or UHF frequency bands, e.g., for the UHF bands approximately 820 to 895 MHz.
At these frequencies, one wavelength can be approximately one foot, thereby allowing great latitude in the design of the antenna system.
Until recently, however, there have been generally only two basic designs of mobile communications antennas for mounting on a non-conductive surface of a vehicle being used.
The most popular of these two designs is a two element antenna having its radiating elements essentially collinear and separated by an open air helical inductor coil. This prior art design yields about 3 dB gain for the signal.
The second major design in current use has two radiating elements with an electrical length equal to substantially one-quarter wavelength and electrically connected into a vertical dipole configuration.
Since any antenna system that involved transmission of its signal through a non-conducting surface involves loss of signal strength and an increase in the standing wave ratio on the tranmission line, a design was sought that would yield a higher signal gain over existing designs, while still minimizing both the signal loss and the standing wave ratio on the transmission line.
The present invention meets these requirements for a higher signal gain over existing designs while still minimizing both the signal loss due to the transmission of the signal through the non-conductive mounting surface and lessening the standing wave ratio, generally found for such designs, on the transmission line.